Every year, around this time, players advertise themselves as being in the best shape of their lives, either because their offseason conditioning regimens improved, or because previously they were lazy. I think the players are almost always being sincere ā they probably, genuinely, feel great ā but as fans, we identify this as a cliche, and we generally dismiss it. For one thing, we hear this claim entirely too often. For another, itās never been demonstrated that thereās a relationship between best shape and on-field success. Or, if you prefer, on-field improvement. Itās been studied, albeit not exhaustively so.
Thereās something particular I want to examine, though, and it has to do with Jhonny Peralta. This is an article about Peralta from Friday morning. Within:
Fewer pounds would be preferable, they told him, as he headed home for the winter. He got the message and lost 18.
āItās good,ā Peralta said. āIāve never been at this weight since Iāve been with Detroit. I finished last season at 236. Iām at 218 now.ā
Peralta continues:
āDombrowski and everybody wanted me working on my range,ā he said. āThatās why I tried to get lighter. So I can be quicker, especially up the middle.ā
Itās funny ā everyone, a year ago, was prepared for the Tigersā infield defense to be dreadful, what with having Prince Fielder and Miguel Cabrera at the corners. It was pretty bad, especially if you ask Rick Porcello (off the record). But at least according to the numbers, Jhonny Peralta was fine. DRS characterizes him as being about a league-average defensive shortstop. UZR characterizes him as being an above-average defensive shortstop. People talk about Peraltaās limited range, but overall, it doesnāt seem like thereās much to complain about. But we can proceed anyway.
Peralta wants to get better at defense, so he says he lost weight. A pretty significant amount of weight, for that matter, and he says he lost it in a healthy way. Itās intuitive how weight loss and better conditioning could make a defender improve. But I was curious whether we could find anything in the numbers if we looked at previous examples. So I pulled up some previous examples.
I identified 16 players who, between 2011 and 2012, claimed to have lost at least ten pounds. They lost this weight on purpose, and it wasnāt because they were ill or anything. I wanted to know what happened with their defense, so I put their season-before and season-after Fld + Pos numbers over a denominator of 1,000 innings. In this way, I get UZR, and I also account for changes in position. A year ago, did weight loss lead to defensive improvement?
Player
Pre-loss
Post-loss
Casey McGehee
6.9
-2.8
Evan Longoria
10.9
-3.7
Ian Desmond
1.1
9.3
Jason Heyward
3.5
10.8
Jay Bruce
-5.7
-9.2
Justin Morneau
-3.1
-11.8
Justin Smoak
-10.8
-7.7
Mark Reynolds
-21.8
-16
Mark Teixeira
-2.6
1.2
Marlon Byrd
3.9
0.0
Miguel Cabrera
-12
-6.4
Mike Moustakas
2.2
14.5
Nelson Cruz
-12.1
-8.2
Nick Swisher
-0.2
-3.9
Shin-Soo Choo
-2.4
-17.9
Torii Hunter
-7.7
3.6
AVERAGE
-3.1
-3.0
Welp. Hard to get less meaningful than that. Some players got better, and some players got worse, but, overall, there was virtually no change within the group. What they were, defensively, in 2011, they were again defensively in 2012.
Now for the caveats. For one thing, you know about the issues with UZR. For another thing, you know about the issues with a small sample size. And what we donāt know is what these players wouldāve done in 2012 if they hadnāt lost weight and trained as they did. We donāt have a control, so it could be that the -3.0 post-loss average is significant. All the players got a year older, and as players get older, defense declines. As a group, the defense didnāt decline, so thatās of interest. But we definitely donāt see a strong effect. If thereās a benefit, weāre talking about a very small handful of runs, probably.
Something to consider is the somewhat arbitrary nature of reporting baseball player weight. Itās not always listed accurately, and sometimes players report weight loss from the previous spring, while sometimes they report weight loss from the end of the year. Additionally, offseason conditioning matters only so much ā all players train in spring training, and then weight can change over the course of the regular season. A player whoās lost, say, 15 pounds come the beginning of February could see his weight change again in the coming months.
Just for the heck of it, how about the players who, between 2011 and 2012, added a significant amount of weight? The table is the same as the table above.
Player
Pre-gain
Post-gain
Brandon Inge
4.1
15.6
Carlos Gonzalez
-4.1
-12.7
Darwin Barney
7.1
11.9
Dexter Fowler
-3.7
-11.9
Elvis Andrus
10.6
11.5
Franklin Gutierrez
21.6
-23.9
Freddie Freeman
-17.7
-11.6
Pablo Sandoval
13.9
2.8
AVERAGE
4.0
-2.3
You could say there might be something there. All the same caveats apply, though. Itās UZR, the sample is even smaller, the players got a year older, and the whole sample might be thrown off by Franklin Gutierrez, who didnāt play that many innings in 2012. Gutierrez was also recovering from an illness, although he was more ill in 2011, when his defense was great. If you donāt trust the Gutierrez numbers, and you remove them, the average numbers balance out. If you leave them in, you get precisely this table. Itās included more for curiosity than for analysis. We need more data, and Iām thinking about collecting it.
Jhonny Peralta wants to get better in the field, so he lost a lot of weight. Thatās good ā itāll make him healthier, and it could improve his durability. But we donāt have much evidence to suggest heāll take a statistical step forward, and if you look at players who gain weight, the data also isnāt conclusive. We do need a lot more of it, however, so thatās something to keep in mind. As intuitive as it is that being in better shape could make your defense better, in reality weāre probably talking about very slight differences, if any differences at all. Whatās hugely critical for defense are instincts and first steps and hand-eye coordination and arm. Losing weight could make you a little quicker, but probably not significantly so, and in Peraltaās case, itās unlikely a matter of 18 pounds could make him more able to get to a bunch of grounders in the hole. Peraltaās range has only a little to do with his physical frame, or at least the parts of his physical frame he can change.
Itās good that Peraltaās in better shape. Maybe heāll keep that up all season. Maybe itāll have real, meaningful benefits. I just wouldnāt expect him to channel his inner Andrelton Simmons. What Jhonny Peralta was, Jhonny Peralta probably will be.

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